Pedicle Screw Extension for Use in Percutaneous Spinal Fixation

ABSTRACT

A bone screw extension assembly for use in placing a connecting rod percutaneously into tissue of a patient comprises a multi-axial bone screw and a screw extension releasably coupled to yoke of the bone screw. The screw extension includes coaxially disposed outer sleeve and inner sleeves that are rotatable relative to each other. A screw engaging member projects radially from the distal end the inner sleeve and is configured to engage an extension coupling surface on the outer sleeve upon relative. A securement member including cooperative surfaces is defined adjacent the distal end of the inner and outer sleeves that is operable upon relative radially secure the sleeves together to thereby maintain the engagement of the screw engaging member and the bone screw.

BACKGROUND

The present disclosure contemplates instrumentation and procedures forachieving spinal fixation or more particularly for percutaneouslyintroducing a spinal fixation system into a patient.

A typical spinal fixation system 10 as shown in FIG. 1 spans betweensuccessive vertebrae V of the spine. An elongated member, such as rod12, extends along the length of the spine and provides an anchor pointfor connecting each vertebra to the rod. The rod is typically contouredto approximate the normal curvature of the spine for the particularinstrumented spinal segments, which may include lordosis or kyphosis.Anchor devices 15 are provided for connecting the vertebral segments tothe elongated member. These anchor devices may include hooks, bolts,screws or other means for engaging a vertebra. For the purposes of thepresent discussion, the anchor device 15 is a bone screw assembly, suchas the screw assembly shown in FIG. 2. However, it should be appreciatedthat the instrumentation and procedures disclosed herein may beimplemented with other types of anchor devices, such as a hook engagedto the lamina of a vertebra for instance.

The bone engaging fastener or screw assembly 15 includes a shank 16 thatcarries threads configured to engage vertebral bone. For instance, thefastener is a pedicle screw with a shank that is threaded for engagementwithin the pedicle of the vertebra. The screw assembly further includesa head 16 a by which the screw, and ultimately the vertebra, is fastenedto the spinal rod 12. In particular, the head 16 a supports a yoke 17that is generally U-shaped to receive the spinal rod therethrough, asdepicted in FIG. 2. The rod 12 may be supported in part by a collar 18mounted over the head 16 a of the bone screw. A cap 19 carries a setscrew 20 that locks the rod within the yoke 17 and thus fastens the rod12 to the bone screw.

One embodiment of a bone screw assembly 15 is disclosed in co-pendingU.S. application Ser. No. 11/762,898 (the '898 Application), entitled“Multi-Axial Fixation Assembly”, filed on Jun. 14, 2007 and published asNo. 2008/0119858, the disclosure of which is incorporated herein byreference. For the purposes of the present disclosure, the bone screw 15may be constructed as disclosed in the '898 Application, although it isunderstood that other bone screw or multi-axial fastener configurationsmay be implanted using the instruments and procedures disclosed herein.In the multi-axial bone screw assembly 15 the yoke 17 is articulatinglyattached to the threaded bone screw 16, and more specifically to thehead 16 a of the bone screw, so that the yoke 17 can adopt a range ofspherical angles relative to the bone screw. Thus, the yoke canarticulate relative to the bone screw fastened in the vertebra so thatthe slot 42 can be aligned to receive the connecting rod 25.

While in the past spinal fixation systems have been implanted in openprocedures involving relatively large incisions through the patient'stissue with significant muscle retraction, more recent procedures havebeen developed to percutaneously introduce spinal fixation systems in aminimally invasive manner. One technique known as the Sextant® System isdescribed in U.S. Pat. No. 6,530,929, issued to Justis, et al. In the'929 patent, separate incisions are made for introducing respectivepedicle screws each attached to a tubular extension extending outwardlyfrom the patient through each incision. A pivot arm coupled to theextensions introduces an elongate rod through another separate incisionremote from the incisions receiving the extensions. The pivot arm urgesthe rod beneath the skin and into the pedicle screws for fixation. Otherpercutaneous systems such as that shown in U.S. Pat. No. 7,306,603issued to Boehm, Jr. et al. utilize tubular pedicle screw extensions toplace a rod longitudinally through the extension into one of the pediclescrews. The rod is then pivoted about the pedicle screw through anincision between the pedicle screws to the second pedicle screw. Othersstill employ systems such as that shown in U.S. Pat. No. 7,250, 052issued to Landry et al. wherein slots in the screw extensions are usedto guide a rod between the extensions through a single incision intoposition in two or more pedicle screws.

Nevertheless, there is current desire for minimally invasive instrumentsand procedures for the percutaneous placement of spinal fixation systemsthat are relatively simple and easy to use and that provide for enhancedassurance of rod introduction and connection to the spinal implants.

SUMMARY

In one aspect, a bone screw extension assembly is provided for use inplacing a connecting rod percutaneously into tissue of a patient,comprising a multi-axial bone screw including a shaft having a threadedscrew portion and a yoke articulatingly attached to the shaft, the yokehaving an exterior surface and a slot therethrough for receiving aconnecting rod, the slot defined by a pair of upstanding arms havingopposed interior surfaces, the yoke including an extension couplingsurface. The assembly further comprises a screw extension releasablycoupled to yoke of the bone screw, the screw extension including anelongate hollow outer sleeve having a distal end and a proximal end andan elongate hollow inner sleeve disposed within the outer sleeve andhaving a distal end and a proximal end, the outer sleeve and the innersleeve being movable rotationally relative to each other

In a further feature, the assembly includes a screw engaging memberprojecting radially from the distal end of one of the outer sleeve andthe inner sleeve and configured to engage the extension coupling surfaceupon relative rotation of the inner sleeve and the outer sleeve; and asecurement member including cooperative surfaces defined adjacent thedistal end of the inner sleeve and the outer sleeve operable uponrelative rotation of the inner sleeve and the outer sleeve to radiallysecure the inner sleeve and the outer sleeve together and to maintainthe engagement of the screw engaging member and the bone screw.

In a further aspect, a combination for use during spinal surgerycomprises a multi-axial bone screw including a shaft having a threadedscrew portion, a screw head and a yoke articulatingly attached to thescrew head, the yoke having an exterior surface and a slot therethroughfor receiving a connecting rod, the slot defined by a pair of upstandingarms having opposed interior surfaces, the yoke having an extensioncoupling surface thereon. The combination further comprises an elongatehollow screw extension having a distal end and a proximal end, theextension including a screw engaging member at the distal end thereof inengagement with the extension coupling surface on the yoke, theextension having a pair of opposing extension slots opening at thedistal end of the extension aligned with the yoke slot and extending foran extent toward the proximal end of the extension, the extensionincluding on an outer surface thereof a persuader coupling member.

In another aspect, the combination includes a rod persuader including anelongate hollow sleeve extending over the outer surface of the screwextension and releasably couplable to the persuader coupling member, thepersuader sleeve having a distal end defining a rod engaging surfacedisposed in a first position to allow a rod to be introduced through theopposing slots in the screw extension and operably movable incooperation with the persuader coupling member to a second positionwherein the rod engaging surface extends over at least a portion of theextension slots to urge the rod into the slot of the yoke.

In a further combination for use during spinal surgery, a multi-axialbone screw is provided including a shaft having a threaded screwportion, a screw head and a yoke articulatingly attached to the screwhead, the yoke having an exterior surface and a slot therethrough forreceiving a connecting rod, the slot defined by a pair of upstandingarms having opposed interior surfaces, the yoke having an openingcommunicating with the screw head and including an extension couplingsurface on the interior surfaces of the upstanding arms. The combinationfurther comprises an elongate hollow screw extension having a distal endand a proximal end, the extension having a rotationally movable screwengaging member projecting radially outwardly from the distal endthereof in engagement with the extension coupling surface of the yokearms.

In another aspect, the combination further comprises a screw driverincluding an elongate inner shaft having a distal end and a proximal endand an outer retention sleeve disposed about the inner shaft and havinga distal end and a proximal end, the inner shaft being freely rotatablewithin the outer retention sleeve and having at its distal end a tipextending through the yoke opening in releasable engagement with thehead of the bone screw for joint rotational movement therewith, thedistal end of the outer retention sleeve being releasably joined to theyoke for joint movement therewith.

DESCRIPTION OF THE FIGURES

FIG. 1 is a representation of a portion of a patient's spineinstrumented with a multi-level fixation system.

FIG. 2 is a perspective view of a bone engaging fastener in the form ofa pedicle screw suitable for use with the instrumentation and proceduresdisclosed herein.

FIG. 3 is a perspective view of instrumentation disclosed herein used tointroduce an elongated connecting element to a fixation assembly.

FIG. 4 is an exploded perspective view of a bone screw and a screwextension assembly disclosed herein.

FIG. 5 is an enlarged view of the bone screw and the distal end of thescrew extension assembly shown in FIG. 4.

FIG. 6 is a cross-sectional view of the bone screw and screw extensionassembly shown in FIG. 5 with the screw extension assembly in a firstposition.

FIG. 7 is a cross-sectional view of the bone screw and screw extensionassembly shown in FIG. 6 with the screw extension assembly mounted onthe bone screw in the first position.

FIG. 8 is a perspective view of the bone screw and screw extensionassembly shown in FIG. 7.

FIG. 9 is an enlarged cross-sectional view of the bone screw and screwextension assembly shown in FIG. 4 with the screw extension assemblymounted on the bone screw in a second position.

FIG. 10 is a perspective view of the bone screw and screw extensionassembly shown in FIG. 9.

FIG. 11 is an enlarged cross-sectional view of the bone screw and screwextension assembly shown in FIG. 4 with the screw extension assemblymounted on the bone screw in a third position.

FIG. 12 is an enlarged exploded view of the proximal end of the screwextension assembly and the socket driver shown in FIG. 4

FIG. 13 is an enlarged perspective view of the distal end of the screwextension assembly shown in FIG. 4 with the assembly in a first loadingposition.

FIG. 14 is a cross-sectional view of the socket driver mounted to thedistal end of the screw extension assembly in a first position.

FIG. 15 is an enlarged perspective view of the distal end of the screwextension assembly shown in FIG. 4 with the assembly in a second loadingposition.

FIG. 16 is an enlarged perspective view of the distal end of the screwextension assembly shown in FIG. 4 with the assembly in a lockedposition.

FIG. 17 is an enlarged cross-sectional view of one embodiment of thedistal end of the screw extension assembly shown in FIG. 4.

FIG. 18 is a perspective view of the bone screw and screw extensionassembly with a screw driver mounted thereon.

FIG. 19 is a cross-sectional view of the bone screw, screw extensionassembly and screw driver shown in FIG. 18.

FIG. 20 is a perspective view of the rod introducer assembly andconnecting rod shown in FIG. 3.

FIG. 21 is a cut-away view of the rod introducer assembly shown in FIG.20 with the connecting rod engaged thereto and the assembly in a lockedposition.

FIG. 22 is a cut-away view of the rod introducer assembly and rod shownin FIG. 21 with the assembly with a first locking mechanism released.

FIG. 23 is a cut-away view of the rod introducer assembly and rod shownin FIG. 21 with the assembly with a second locking mechanism releasedand the rod disengaged from the assembly.

FIG. 24 is an enlarged perspective view of the distal end of the rodintroducer assembly shown in FIG. 20 with the rod disengaged from theassembly.

FIG. 25 is an enlarged perspective view of the distal end of the rodintroducer assembly shown in FIG. 20 with the rod engaged to theassembly.

FIG. 26 is an enlarged cross-sectional view of the rod engaged to theassembly as shown in FIG. 25.

FIG. 27 is an enlarged view of the second locking mechanism of the rodintroducer assembly shown in FIG. 20 with the mechanism in a lockingposition.

FIG. 28 is an enlarged view of the second locking mechanism of the rodintroducer assembly shown in FIG. 20 with the mechanism in a releaseposition.

FIG. 29 is a cross-sectional view of a rod detector assembly for usewith the instruments and procedures disclosed herein, shown with thedetector flag in a first position.

FIG. 30 is an enlarged cross-sectional view of the distal end of the roddetector assembly shown in FIG. 29.

FIG. 31 is a cross-sectional view of the rod detector assembly shown inFIG. 29 with the detector flag in a second indicator position.

FIG. 32 is an enlarged cross-sectional view of the proximal end of therod detector assembly shown in FIG. 29.

FIG. 33 is a perspective view of the screw extension assembly, rodintroducer assembly and rod detector assembly in one position during aprocedure disclosed herein.

FIG. 34 is a view of a rod introducer assembly and screw extensionassembly with a rod disposed therein, prior to mounting the introducerassembly on the extension assembly.

FIG. 35 is an enlarged cross-sectional view of the proximal end of therod introducer assembly mounted on the screw extension assembly with theintroducer assembly in a first position.

FIG. 36 is an enlarged view of the distal end of the rod introducerassembly in the first position mounted on the screw extension assembly.

FIG. 37 is an enlarged cross-sectional view of the proximal end of therod introducer assembly shown in FIG. 35 with the introducer assembly ina second position.

FIG. 38 is an enlarged view of the distal end of the rod introducerassembly in the second position mounted on the screw extension assembly.

FIG. 39 is an enlarged cut-away view of the advancement mechanism of therod introducer assembly shown in FIG. 34.

FIG. 40 is an enlarged cut-away view of the advancement mechanism shownin FIG. 39.

FIG. 41 is a perspective view of a compression/distraction device asdisclosed herein, shown with the jaws open and the fulcrum in a firstposition.

FIG. 42 is a top view of the compression/distraction device shown inFIG. 41.

FIG. 43 is a perspective view of the compression/distraction deviceshown in FIG. 41, shown with the jaws closed.

FIG. 44 is a top view of the compression/distraction device shown inFIG. 43, shown with the jaws closed.

FIG. 45 is a perspective view of a compression/distraction device asdisclosed herein, shown with the jaws open and the fulcrum in a secondposition.

FIGS. 46 a-c are perspective views of alternative fulcrums for use withthe compression/distraction device shown in FIG. 41.

FIG. 47 is a perspective view of the compression/distraction device ofFIG. 41 used in a compression procedure.

FIG. 48 is a perspective view of the compression/distraction device ofFIG. 41 used in a distraction procedure.

FIG. 49 is a perspective view of one step of one procedure disclosedherein.

FIG. 50 is a perspective view of a further step of the procedure.

FIG. 51 is a perspective view of an additional step of the procedure.

FIG. 52 is a perspective view of one step of another procedure disclosedherein.

FIG. 53 is a perspective view of a further step of the procedure.

FIG. 54 is a perspective view of an additional step of the procedure.

FIG. 55 is a perspective view of one step of yet another proceduredisclosed herein.

FIG. 56 is a perspective view of a further step of the procedure.

FIG. 57 is a perspective view of an additional step of the procedure.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

Referring to FIGS. 3 and 4, certain components of the instrumentationdisclosed herein are depicted as used according to certain proceduresdisclosed herein. In particular, three bone screw assemblies 15 areengaged to three vertebrae V in preparation for a multi-level fixationof the spine. An elongate connecting member, such as connecting rod 25,is configured to be received within the yokes 17 of each of the screwassemblies to connect each of the vertebral levels in a conventionalmanner. When the construct is complete, the rod will be locked to eachof the screw assemblies, such as by the cap 19 and set screw 20illustrated in FIG. 2. As shown in FIG. 3, each of the screw assemblies15 carries a screw extension assembly 32 that is sized to be accessibleoutside the patient's skin. The patient's skin or fascia is depicted asa phantom line S for illustrative purposes only, with the understandingthat the level of the fascia relative to the fixation location on thevertebral bodies will vary from patient to patient. The instrumentsfurther include a rod introducer assembly 34 that is used to introducethe connecting rod 25 through and into the yokes 17 of each of the bonescrew assemblies 15. Once the rod is situated within the bone screwyokes, a rod persuader assembly 36 may be used to fully seat the rodtherein for final tightening. The nature and manner of operation ofthese and other instruments are described herein.

Screw Extension Assembly

Details of the screw extension assembly 32 and its interface with thebone screw assembly 15 will be explained with reference to FIGS. 4-17.Looking first at the bone screw assembly 15, and particularly at FIGS.4-7, the yoke 17 includes opposed upstanding arms 40 that are separatedto define a slot 42 therebetween. The slot 42 is sized and configured torelatively snugly receive the connecting rod 25 therein. For some bonescrew assemblies, the connecting rod may be seated within a U-shapedbase of the slot 42. For the present disclosure, the connecting rod isseated on the sleeve 18 rather than at the base of the yoke slot, all inaccordance with the bone screw assembly disclosed in the '898Application incorporated by reference above.

The arms 40 of the yoke 17 include facing interior surfaces 44 whichdefine internal threads 48, as best seen in FIG. 6. The threads 48 areconfigured to mate with the set screw 20 to clamp the connecting rod 25within the yoke and for final fixation of the bone screw assembly, asdescribed in the '898 Application. The upstanding arms 40 furtherinclude an exterior surface 46 that is partially cylindrical and flatside surfaces 47 on opposite sides of the slot 42. The yoke furtherdefines a tool bore 49 aligned with a tool recess 22 at the base of thebone screw head 16 a that is used to drive the bone screw 16 into thevertebral bone.

As thus far described, the yoke 17 is generally similar to the yokes ofother bone screw assemblies, including the bone screw described in the'898 Application. In the embodiment disclosed herein, the interiorsurface 44 of the yoke 17 defines an undercut 50 that forms a couplingsurface 51 at the mouth of the slot 42, as best seen in FIG. 6. Thecoupling surface 51 provides an interface for coupling to the screwextension assembly 32.

The pedicle screw extension assembly 32 includes an elongate hollowouter sleeve 55 having a perimetric sidewall that defines a bore 56extending from a proximal end 55 a to a distal end 55 b. A lower boreportion 56 a of the bore adjacent the distal end 55 b is sized to berelatively snugly received about the exterior surface 46 of the yoke 17as shown in FIG. 7. The outer sleeve further defines a slot 59 throughthe sleeve sidewall adjacent the distal end 55 b of the sleeve andextending across the diameter of the sleeve, as shown in FIG. 4. Theslot 59 is sized to receive a connecting rod 25 therethrough as depictedin FIG. 3. The slot 59, which opens through the distal end 55 b, may belong enough proximally in certain embodiments to extend above the fasciaS so that the connecting rod 25 may be introduced into the screwextension assembly 32 outside the patient, as explained in more detailherein.

Returning to FIGS. 5, 6, the extension assembly also includes anelongate hollow inner sleeve 57 concentrically and rotatably disposedwithin the bore 56 of the outer sleeve 55. The inner sleeve has aperimetric sidewall that defines a central bore 58 from a proximal end57 a (see FIG. 13) to a distal end 57 b that is configured for passageof other instruments as described herein. The inner sleeve furtherdefines a slot 67 opening through the sleeve sidewall at the distal end57 b of the inner sleeve that is generally coincident in length andwidth with the slot 59 of the outer sleeve. The inner sleeve 57 isrotatable relative to the outer sleeve 55 between a first position shownin FIG. 5 in which the inner sleeve 57 essentially covers or closes theslot 59 in the outer sleeve, and a second position illustrated in FIG. 8in which the two slots 59 and 67 are aligned so that a connecting rodcan be pass through the screw extension assembly 32.

FIGS. 5-11 show the screw extension assembly 32 in various stages ofrelative movement between the outer and inner sleeves 55, 57 to engagethe yoke 17 of the bone screw assembly 15. In FIGS. 5 and 6, the screwassembly 15 is shown just prior to contact with the screw extensionassembly. The outer and inner sleeves are in the first positiondescribed above in which the inner sleeve 57 covers or closes the slot59 in the outer sleeve 55. The yoke 17 of the bone screw assembly isaligned so that the upstanding arms 40 are aligned with the slot 58 inthe inner sleeve 57. The flat side faces 47 are thus aligned to passinto the slot 58 in a close fit.

In FIG. 7 the yoke 17 is fully seated within the screw extensionassembly 32. More specifically, the proximal end 17 a of the yoke isseated against the yoke mating surface 76 at the base of the lower boreportion 56 a of the bore 56 in the outer sleeve 55. This lower portion56 a may further define flat surfaces 72 to align the flat side faces 47of the yoke 17 as the yoke advances into the lower bore portion 56 a. Itcan thus be appreciated that once the yoke 17 is fully seated within thelower bore portion 56 a of the outer sleeve the yoke and outer sleevewill rotate and pivot together. More importantly, the outer sleeve willhold the yoke while the inner sleeve rotates relative to both componentsto firmly engage and lock the yoke to the screw extension assembly.

In order to effect this engagement, the inner sleeve 57 is provided witha yoke engagement member 60 at the distal end 57 b of the sleeve. Theyoke engagement member 60 includes generally radially outwardly directedflanges 61 that interface with coupling surfaces 51 defined by undercuts50 at the proximal end 17 a of the yoke, as seen in FIG. 6. As shown inFIG. 7, when the yoke is seated within the outer sleeve, the yokeengagement member 60 of the inner sleeve 57 is aligned with the couplingsurfaces 51 of the yoke 17. The radial flanges 61 are initially situatedwithin flange recesses 71 defined in the outer sleeve 55. From thisposition the inner sleeve 57 may be rotated relative to the outer sleeve55 and to the yoke 17 connected to the outer sleeve. The effect of thisrelative rotation is illustrated in FIGS. 8-11. In FIGS. 8-9 the innersleeve 57 is shown at the beginning of this relative rotation. As bestseen in FIG. 9, as the inner sleeve rotates the radial flanges 61 areguided by the flange recesses 71 beneath the undercuts 50 and intoengagement with the coupling surfaces 51 of the yoke proximal end 17 a.The radial flanges 61 and the undercuts 50 are configured so thatcontinued rotation of the inner sleeve relative to the yoke tends topull the yoke upward or proximally toward the yoke mating surface 76 ofthe outer sleeve, as shown in FIGS. 10-11. In this position the rodslots 59 and 67 are aligned and the screw extension assembly isessentially supported by the bone screw assembly, which is itselfsubsequently anchored to the vertebra. The yoke engaging flange 61 andundercut 50 may be configured to provide a tighter fit as the innersleeve is rotated relative to the outer sleeve. This may beaccomplished, for instance, by increasing the thickness of the radialflange 61 radially outwardly in an upward angle and forming the undercut50 to have a complementary configuration to accommodate the increasedthickness of the radial flange 61 around the circumference of thecoupling surface.

The screw extension assembly 32 may incorporate additional features toensure a tight engagement between the extension assembly and the bonescrew assembly 15 or yoke 17. Referring to FIGS. 5 and 9, the innersleeve 57 may incorporate a securement member 64 that is configured toengage a securement recess 73 in the outer sleeve 55. The securementmember may include a downwardly or distally projecting securement flange65 that is received within an upwardly opening flange groove 74, as bestseen in FIG. 9. Like the interface between the yoke engaging flange 61and undercut 50, the securement flange 65 and flange groove 74 may beconfigured to provide a tighter fit as the inner sleeve 57 is rotatedrelative to the outer sleeve 55. Thus, the width of the flange 65 may beincreased along the circumference or the width of the groove 74decreased along the circumference so that the fit becomes tighter as theinner sleeve approaches the second position shown in FIG. 11. Uponrotation of the outer and inner sleeves 55, 57, securement flange 65extending into flange groove 74 also serves to minimize or preventoutward radial splaying of the outer and inner sleeves 55, 57.

The combination of the yoke engaging member 60 and the securement member64 of the inner sleeve and the interface of these elements to the yokeand outer sleeve, respectively, allows the screw extension assembly 32to be firmly fastened to the yoke 17 and screw assembly 15 when the bonescrew 16 is threaded into a vertebra. The screw extension assembly 32may be manipulated or articulated relative to the bone screw 16. The rodslots 59 and 67 will thus always be aligned with the slot 42 in the yoke17 of the bone screw assembly to facilitate placement of the connectingrod 25, as described herein.

In the illustrated embodiment, the yoke engagement member 60incorporates a radially inwardly directed flange 61 while the yoke 17incorporates a radially formed coupling surface 51 and undercut 50.Alternatively, these features may be reversed between the inner sleeveand yoke so that the yoke 17 incorporates a radially outwardly directedflange that mates with a radially inwardly formed groove in the distalend 55 b of the inner sleeve 55. Similarly, the securement member 64 ofthe inner sleeve 57 and the securement recess 73 of the outer sleeve 55may be reversed or re-oriented.

As thus far described it can be seen that the operation of the screwextension assembly 32 relies upon rotation of the inner sleeve relativeto the outer sleeve. In one aspect of the assembly 32, the proximal end55 a of the assembly is configured to accept a socket driver 38, asshown in FIG. 4. The structure and operation of the socket driver isshown in more detail in FIGS. 12-17. The socket driver 38 includes agenerally cylindrical socket 80 with a driver socket 82 formed in thebase of the rectangular socket and a generally rectangular rim 81 formedat the distal opening of the cylindrical socket 80. A spindle 84 isprovided for connection to a driving tool for rotating the socket driver38 or to provide a gripping interface to manually rotate the socketdriver. The rectangular rim 81 is configured to engage the generallyrectangular outer surface 86 at the proximal end 55 a of the outersleeve 55. When the rim 81 is in contact with the outer surface 86 thesocket driver 38 cannot be rotated relative to the outer sleeve 55. Inthe illustrated embodiment the mating surfaces of the rim and outersurface are generally rectangular, although other configurations arecontemplated that prevent relative rotation between the socket driverand the outer sleeve.

However, the outer sleeve 55 further defines a radially inward groove 87defined below or distal to the rectangular surface 86. This groove 87 isarranged to be aligned with the rectangular rim 81 when the socketdriver 38 is fully seated on the proximal end 55 a of the outer sleeve55, as depicted in FIG. 14. Thus, when the end of the outer sleeve isadjacent the end of the cylindrical socket 80 the rim 81 is aligned withthe groove 87. In this position, there is no surface of the outer sleevethat bears against the rectangular surface of the rim 81 so the socketdriver 38 is free to rotate relative to the outer sleeve 55.

The driver socket 82 is configured to engage the proximal end 57 a ofthe inner sleeve 57. In particular, the proximal end 57 a includes amating end 88 that is complementary to the driver socket 82. In oneembodiment, the driver socket and mating end have a hex configuration sothat the socket driver 38 can be used to rotate the inner sleeve 57 whenthe mating end 88 is disposed within the driver socket 82, as shown inFIG. 14. In a particular configuration the driver socket 82 may define a12-point contact socket so that in combination with the rectangularouter surface 86 at the proximal end 55 a of the outer sleeve 55, thesocket driver 38 may be engaged every ninety degrees.

The screw extension assembly 32 may incorporate features to preventrelative rotation between the inner and outer sleeves. For instance,when the screw extension assembly is engaged to a bone screw assembly itis desirable to ensure that the two assemblies are locked and cannot beinadvertently disengaged. Since engagement or disengagement occurs withrelative rotation between the inner and outer sleeves, preventinginadvertent rotation of the inner sleeve can prevent inadvertentdisengagement from the screw assembly 15. Accordingly, the screwextension assembly includes a displaceable retention ring 90 thatinitially engages the mating end 88 of the inner sleeve 57. Theretention ring 90 may include a hex interface 90 a for engaging the hexfeatures of the mating end. The retention ring 90 is held againstrotation relative to the outer sleeve, while permitting axial movementof the ring within the outer sleeve. Thus, the retention ring may defineone or more longitudinally extending capture slots 91 that receive acorresponding capture pin 92 that is embedded in the outer sleeve asshown in FIG. 12. The retention ring 90 is thus permitted to slideaxially or longitudinally within a bore 94 at the proximal end 55 a ofthe outer sleeve 55 from the extended position shown in FIG. 12 to adepressed position shown in FIG. 14. A biasing spring 93 is disposedwithin the bore 94 to bias the retention ring 90 to the extendedposition in which the retention ring engages the hex end 88 of the innersleeve 57, as described above.

As shown in FIGS. 14-15, the retention ring 90 can be moved to itsdepressed position by pressing the socket drive 38 downward or towardthe proximal end 55 a of the outer sleeve. The base of the cylindricalsocket 80 contacts the retention ring 90 pushing it down with the socketdriver until the cylindrical socket bottoms on the top of the outersleeve. In this position the retention ring 90 is clear of the hex end88 so that the hex end is free to be rotated by the hex socket 82. (Asexplained above, in this position shown in FIG. 14 the rectangular rim81 is also clear of the rectangular outer surface 86 of the outersleeve).

The screw extension assembly 32 further includes an indicator 95 thatindicates to the surgeon the relative position of the inner and outersleeves. Thus, when the screw extension assembly 32 is in its initialorientation (i.e., with the inner sleeve in the position shown in FIG. 5to accept a bone screw yoke) the indicator includes the indicia 95 a“LOAD” viewable in the window 95 c formed in the outer sleeve. Theindicia 95 a is affixed or applied in a suitable manner to the outersurface of the inner sleeve. When the screw extension assembly 32 hasbeen coupled to the yoke 17 of the bone screw assembly (as shown inFIGS. 10-11) the indicia 95 b “LOCKED” is visible through the window 95c, as illustrated in FIG. 16. As shown in FIG. 4, this indicator 95 isat the proximal end 55 a of the outer sleeve so that it is readilyvisible to the surgeon outside the surgical site.

In an alternative embodiment a modified retention ring 90′ is operableto free the inner sleeve for rotation relative to the outer sleeve, asillustrated in FIG. 17. In this embodiment, a number of retention balls92′ are situated between a locking bore 97 defined in the outer sleeve55 and a corresponding number of ball recesses 98 defined in the innersleeve 57. The retention ring 90′ is initially positioned as shown inFIG. 17. When the ring is pushed in the direction of the arrow R a lowercam surface 96 contacts and bears against the retention balls 92′. Thiscontact gradually pushes the retention balls 92′ radially inward in thedirection of the arrow B to a release position in which the balls areseated within the corresponding recesses 98. In this position the innersleeve 57 is free to rotate relative to the outer sleeve 55. A biasingspring 93′ may be provided to bias the retention ring 90′ away from therelease position and to the locked position in which relative rotationis prevented.

In one embodiment, the socket driver 38 may be provided with a steppedshaft 89′ extending from the socket hex 82 (FIG. 14) and projectingthrough the inner sleeve 57 as shown in FIG. 17. The stepped shaft 89includes a stepped distal end 89′b that is sized to be retained by thecapture balls 92′ when the socket driver 38 is fully seated on the innersleeve and has fully depressed the retention ring 90′. The capture balls92′ thus prevent removal of the socket driver as long as they are in theinboard position denoted by the arrow B.

Screw Driver Instrument

The screw extension assembly 32 is configured to accept additional toolsfor access to the bone screw assembly. For instance, the bore 58 of theinner sleeve 57 is sized to receive a screw driver 100 as shown in FIGS.18-19. The screw driver 100 includes at the proximal end a handle 101connected to a shaft 102 to permit manual rotation of the shaft. Theshaft 102 includes at the distal end a tip defining an engagement end103 that is configured to engage a drive tool recess 22 in the base ofthe bone screw head 16 a. The engagement end and drive tool recess canbe configured in a conventional manner, such as with a hex or Torxfeature. The shaft 102 is sized so that the engagement end 103 can bereceived within the recess 22 while the handle 101 is accessible at theproximal end of the screw extension assembly 32.

The screw driver tool 100 includes an outer retention sleeve 104 havingan interior bore 104 a through which the shaft 102 extends. The shaft102 and retention sleeve 104 are coupled to each other to allow freerelative axial and rotational movement therebetween. The distal end 103a of the retention sleeve 104 is provided with exterior threads to matchthe internal threads 48 on the interior surfaces 44 of yoke 17. Theretention sleeve 104 is connected to a knob 106 (FIG. 18) situated on oradjacent the proximal end of the screw extension assembly 32 that isconfigured to facilitate manual rotation of the retention sleeve tothread the distal end 103 a into the yoke. A stop 108 is rotatablymounted on the shaft 102 and is configured to seat within the slot 42 ofthe yoke 17 to support the shaft and retain the sleeve. Upon threadedconnection of the outer retention sleeve 104 to the yoke 17, theretention sleeve 104 bears against the stop 108 and the stop bearsagainst the yoke to provide joint rotational movement of the retentionsleeve, stop and yoke. Prior to such threaded connection, the engagementend 103 of the inner shaft is guided into the drive tool recess 22 inthe base of the bone screw head 16 a. The stop 108 may be sized toprevent threading of the retention sleeve into the yoke unless and untilthe end 103 of the shaft is engaged within the tool recess of the bonescrew. Once the tool 100 is properly seated, rotation of the handle 101that is connected to the shaft 102 will rotate the bone screw shank 16.With the screw extension assembly 32 and the retention sleeve 104attached to the yoke for joint movement, and with the yoke 17 being ableto freely articulate with respect to screw shank 16, the screw extensionassembly 32 may be manually held while the handle 101 is rotated todrive the screw shank 16 into a pedicle of a vertebra.

The screw extension assembly thus provides an avenue for guiding thescrew driver instrument 100 into engagement with the bone screw. Even ifthe screw extension assembly is articulated relative to the bone screw,a minor manipulation of the assembly will automatically align the screwdriver instrument with the drive tool recess. Once engaged the screwdriver can be used to thread the bone screw 16 into the vertebra in aknown manner and then removed from the screw extension assembly. Theshaft 102 of the screw driver 100 may be provided with a guide wirelumen 107 to allow introduction of the tool over a previously positionedguide wire.

Rod Introducer Assembly

With the bone screw assemblies anchored in the vertebrae with the screwextension assemblies engaged to the screw assemblies, the connecting rod25 can be introduced through the rod slots 59, 67 in the extensionassemblies using a rod introducer assembly 34, as shown in FIG. 3.Details of the rod introducer assembly and its operation are shown inFIGS. 20-28. The introducer assembly 34 includes a handle 110 configuredto be manually grasped to manipulate the connecting rod 25 attached tothe introducer assembly. The handle is also configured for easy accessto the actuation mechanism 112 used to enable grabbing and locking aconnecting rod to the assembly, as well as to push buttons 145 and 152used to release the actuation mechanism in various stages of operation,as explained herein. The handle 110 and lever 113 of the actuationmechanism 112 may be particularly configured to permit one-handedoperation of the lever during its stages of actuation.

Looking first at FIG. 26, the connecting rod 25 includes an introductionend 27 that may be tapered to facilitate introduction of the rod throughtissue, an incision, and/or the rod slots 59 and 67 in the screwextension assemblies. The elongated body 26 of the rod is sized to spanthe distance between the instrumented vertebrae and may have a curvaturecalibrated to accommodate or correct the orientation of the instrumentedvertebral levels in a known manner, such as for lordosis and kyphosis.The connecting rod further includes an engagement end 28 that defines anopening 29 and a series of flats 30 a-30 e. These features of theengagement end 28 provide the interface with the rod insertion assembly34.

Turning to FIGS. 20-21, the rod insertion assembly 34 includes an outersleeve 114 extending from the handle 110. As shown in FIG. 3 the outersleeve has a length approximating the height of the extension assemblies32 above the bone screws mounted in the vertebrae. The outer sleeve hasa length sufficient for the surgeon to manipulate the handle 110 outsidethe patient while the connecting rod 25 carried by the instrument isfully seated within the yokes of the bone screw assemblies. The outersleeve 114 is at least generally tubular along a portion of its lengthand it thus hollow to slidably receive an inner actuator shaft 116 fortranslational movement within. The distal portion of the outer sleeve,which in the illustrated embodiment may constitute about half the lengthof the sleeve, branches into opposed flexible legs 118 separated by anexpandable slot 121 through the top and bottom of the sleeve 114. Thelegs are capable of flexing outwardly relative to each other to form anexpandable opening 119 into which the engagement end 28 of theconnecting rod 25 is introduced (see also FIGS. 24-25). The legs 118 maybe configured to be initially biased together or toward each other, thebiasing force being provided by the natural resilience of the legs.

The expandable slot 121 of the legs defines opposing cam elements 128.The cam elements 128 are configured to provide a reduced slot width witha cam surface 129 (FIG. 25) leading to that reduced width. The inneractuator shaft 116 includes an actuator pin 122 that projectsdiametrically across the outer sleeve 114 (FIG. 21) and is arranged tocontact the cam elements 128 as shown in FIGS. 24 and 25. In theconfiguration shown in FIG. 24 the actuator pin 122 is disposed directlybetween both cam elements 122, widening the gap between the elements,which in turn forcibly deflects the flexible legs 118 apart, and whichultimately increases the size of the expandable opening 119. On theother hand, when the actuator pin 122 is in the position shown in FIG.25, the pin 122 is beyond or distal of the cam elements 128 so that theflexible legs are biased toward each other, thereby decreasing the sizeof the expandable opening 119. In this position, a locking pin 124projecting from the inner actuator shaft 116 engages a recess 127 formedby locking hooks 126 a, 126 b adjacent the distal end 126 of the outersleeve 114. It is noted that each leg includes a locking hook. Thus,when the locking pin 124 is disposed within the recess 127 defined byeach locking hook 126 a, 126 b, the pin prevents separation of thelocking hooks, and consequently separation of the flexible legs 118.Each leg defines a notch, such as notch 118 b to receive the lockinghook of the opposing leg, such as hook 126 a, as shown in FIG. 25.

The flexible legs 118 of the outer sleeve 114 include inwardly directedposts 120 that are sized to be received within the opening 29 in theengagement end 28 of the connecting rod 25, as shown in FIG. 24. Theposts 120 are disposed generally perpendicular to the longitudinal axisof the outer sleeve 114 and are particularly sized so that they provideadequate space between them when the expandable opening 119 is at itslargest extent so that the engagement end 28 of the rod can fit betweenthe posts. The posts are also sized so that they do not contact eachother when disposed within the opening 119, as shown in FIG. 25.(Alternatively the posts may be half cylinders that overlap each otherwithin the opening 119). The posts 120 thus provide structure forengaging the connecting rod 25 and holding it to the rod introducerassembly 34 when the flexible legs 118 are in their closed position. Thelocking pin 124 and locking hooks 126 a, b hold the legs together sothat the connecting rod cannot be removed from the rod introducerassembly 34, at least not without disengaging the locking pin and hooks.

The locking pin 124 and the actuator pin 122 are advanced or retractedby axial movement of the inner actuation shaft 116 within the outersleeve 114. This movement is accomplished by the actuation mechanism112. The lever 113 of the actuation mechanism is coupled to the innershaft 116 by a linkage 133. The lever itself is pivotably mounted to thehandle 110 at pivot pin 132 so that the lever can pivot from the firstposition shown in FIG. 21, to a second position shown in FIG. 22 to athird position shown in FIG. 23. The pivoting movement of the leverabout pivot pin 132 is translated to linear movement of the inner shaft116 through the linkage 133. In the first position shown in FIG. 21, thelever is locked within the handle 110 and the inner shaft has moved toits farthest distal extent. In this farthest distal position, thelocking pin 124 is engaged in the locking recesses 127 and the legs 118are locked together to grip the connecting rod, as shown in FIG. 25. Inthe third position shown in FIG. 23, the lever is fully unlocked fromthe handle and the inner shaft 116 has moved to its nearest proximalextent. In this proximal position, the actuator pin 122 has separatedthe legs 118 so that the connecting rod is automatically disengaged fromthe rod introducer assembly 34, as shown in FIG. 24. In the secondposition, also indicated as a neutral position, shown in FIG. 22, theposts 120 are disposed within the opening in the engagement end 28 ofthe connecting rod so the rod is still held by the rod introducerassembly, but in this position the rod can be pivoted about the posts tovary the angle of the connecting rod relative to the outer sleeve 114 asthe rod is introduced into the surgical site, as explained more fullyherein.

As shown in FIG. 26, the distal end of the inner actuator shaft 116defines a rod engaging end 156. This end 156 defines a series of flatportions 157 a, b, c that generally correspond to the flat surfaces 30a-30 e of the engagement end 28 of the rod. The flat portions of the rodengaging end 156 define a partial polygonal socket which is configuredto complementarily mate with the flat surfaces of the engagement end ofthe rod so that when the rod engaging end 156 is held directly againstthe engagement end 28 of the rod the rod cannot pivot about the posts120. Flat surface 157 b lies transverse, and preferably generallyperpendicular, to and crosses the longitudinal axis of the inner shaft116, with adjacent flat surfaces 157 a and 157 c lying angularly withrespect to surface 157 b and defining the socket therewith. In apreferred arrangement, the socket defined by the flat portions 157 a, b,c of the rod engaging end 156 are the mirror image of the flat surfaces30 a-30 e of the engagement end 28 of the rod. Thus, in the firstposition of the lever 113 shown in FIG. 21 the rod engaging end 156 isin flush contact with the engagement end 28 of the rod. However, in thesecond position of the lever shown in FIG. 22, the inner shaft 116 isbacked off slightly from the engagement end 28 of the rod, as shown inFIG. 26. The gap between the flat surfaces of the engagement end, suchas surfaces 30 c, 30 d and 30 e, and the flat connecting surfaces 157 a-c of the actuator shaft, allows the rod to be pivoted about the posts120. The proximity of the rod engaging end 156 to the engagement end 28of the rod provides resistance to this movement so that the rod can bemoved to a particular angle and held there without any outside force.The resistance provided by the corners of the polygonal socket at therod engaging end 156 acts as detent and is readily overcome by slightmanual pressure which creates tactile feedback to the surgeon and anaudible snapping sound.

The array of flat surfaces 30 a-e at the engagement end 28 of the rod 25allow the rod to be positioned and locked in five angular orientationsrelative to the rod introducer assembly 34. Thus, when the flat surface30 d is aligned with the rod engaging end 156 of the actuator shaft 116,the rod is oriented at a 45 degree angle relative to the outer sleeve114 of the introducer assembly, as shown in FIG. 26. When flat surface30 e is aligned with flat engagement surface 157 b, the rod is at anangle of 90 degrees. When the flat surface 30 c is aligned the rod isgenerally collinear with the outer sleeve, or at an angle of 0 degrees.The rod 25 can also be pivoted downward from the position shown in FIG.26 to the 45 and 90 degree angles. It is noted that when the rod ispivoted to one of the “upward” positions, such as shown in FIG. 26, thecurvature of the rod is generally toward the rod introducer assembly andis used typically for lordotic applications. On the other hand, if therod is reversed to one of the “downward” positions opposite that shownin FIG. 26, the curvature of the rod faces away from the introducerassembly and is used typically in kyphotic applications. This featureprovides flexibility for the surgeon to apply different correctionsusing the same rod and introducer assembly.

It should be understood that the angular configurations of the flatportions 157 a, b, c of the rod engaging end 156 and the correspondingflat surfaces 30 a-30 e of the engagement end 28 of the rod 25 may bevaried to obtain other desired angles, which may, for example be inthirty degree or other increments. In addition, the rod engaging end 156of the inner shaft 116 may be formed to have only a single flat surface,such as surface 157 b to hold tightly against one of the flat surfaces30 a-30 e of the engagement end 28 of the rod 25. It should also beappreciated that the socket defined at the rod engaging end 156 of theinner shaft 116 may be formed of a curved surface to mate frictionallywith a like curved surface formed on the engagement end 28 of the rod25. As such, in the second position as shown in FIG. 22 the gap wouldallow free non-detented pivotal movement of the rod 25 about the posts120 until the shaft 116 is moved axially more distally to cause the rodengaging end 156 to tightly frictionally engage and hold the engagementend 28 of the rod 25 in a position as shown in FIG. 21.

The rod introducer assembly 34 incorporates two locks used to hold theactuation mechanism 112, and particularly the lever 113, in the firstposition (FIG. 21) and in the second position (FIG. 22). As shown inFIG. 21, the first locking mechanism 140 includes a first lockingsurface 141 on the lever 113. A first locking element 143 is slidablymounted within the handle 110 and may be integrated onto a push button145 that is biased outward by spring 146. The first locking surface 141and the first locking element 143 may be in the form of engaging hooks,as illustrated in FIG. 21. The spring 146 biases the push button outwardso that the two hooks remain engaged until the push button 145 isdepressed against the spring.

When the push button 145 is depressed to release the first lockingmechanism 140, the lever 113 pivots slightly upward to the secondposition shown in FIG. 22. The lever is pushed upward, or pivoted aboutthe pivot pin 132 by the post 135. The post 135 is biased toward thelever 113 by a spring 136 and in a direction to cause the lever to pivotabout the pivot pin. It can be appreciated that the lever can bereturned to the first position shown in FIG. 21 by depressing the leverdownward toward the handle, thereby pushing the lever back against thepost 135 and spring 136.

The lever 113 is held in the second position shown in FIG. 22 by thesecond locking mechanism 150, which is shown in more detail in FIGS. 23,27 and 28. The second locking mechanism 150 includes a second lockingsurface 151 integrated into the lever 113. It is noted that FIG. 23 onlyshows one such surface 151 since this figure is a cross-sectional viewthrough the lever. Thus, an additional locking surface 151 is a mirrorimage to the surface depicted in FIG. 23. The second locking surfacesbear against the second locking element 153 mounted within the handle110. As shown in the detail view of FIG. 27, the second locking element153 includes two plates 155 forced apart by a spring 154 interposedtherebetween. Each locking surface 151 of the lever 113 thus fit betweena respective plate 155 and a side wall 111 of the handle 110, asgenerally depicted in FIG. 21. It can be appreciated that the plates 155thus exert a friction force against the second locking surfaces 151 toprevent the lever 113 from pivoting upward.

The two plates 155 are carried by respective release buttons 152 thatproject laterally outward from the handle 110 when the second lockingmechanism 150 is in its locked position illustrated in FIG. 27. Therelease buttons 152 may be depressed inward toward each other, as shownin FIG. 28, to push the plates 155 towards each other against the forceof the spring 154. When the plates are in the position shown in FIG. 28,the contact between the plates and the second locking surfaces 151 ofthe lever 113 is reduced or eliminated so that the lever 113 is free topivot upward to the third position shown in FIG. 23.

Rod Detector

The rod detector assembly 160 is used to detect the presence of anelongated connecting rod 25 into the bone screw assemblies 15 engaged tothe vertebrae, as shown in FIG. 3. However, in certain procedures thepoint of entry of the rod 25 into the rod slots 59, 67 of the screwextension assemblies 32 (as described above) may not be visible to thesurgeon. More particularly, since the rod may percutaneously enter thescrew extension assemblies beneath the fascia S the surgeon may not beable to visually verify that the rod is properly positioned within therod slots 59, 67, and ultimately within the slots 42 in the yokes 17 ofthe bone screw assemblies. A rod detector 160 is provided that canprovide a readily seen and easily discernable visual indicator to thesurgeon above the surgical site, as shown in FIG. 32. Details of the roddetector are shown in FIGS. 29-31.

The rod detector 160 includes a generally tubular body 161 that is sizedto fit within the bore 58 of the inner sleeve 57 of the screw extensionassembly 32, as best seen in FIG. 31. In certain embodiments, thetubular body 161 is open at a slot 162 along a substantial portion ofthe length of the body, as seen in FIG. 30. At the distal end, the bodyforms two diametrically opposed branches 163 that are coincident withthe slot 162 from one side of the body and that form a diametricallyopposite slot. The detector includes a cap 166 affixed to the tubularbody that is sized to seat on top of a screw extension assembly, asdepicted in FIG. 32. The tubular body is sized to extend along asubstantial portion of the length of the inner sleeve 57 but not so faras to interfere with the introduction of the rod 25 into the rod slot 67of the sleeve.

The rod detector 160 includes a flag 164 that projects upward from thecap 166 as shown in the figures. The flag 164 may be connected to orintegral with a strip 165 that spans the length of the tubular body to abase 172 at the top of the opposed branches 163, as seen in FIG. 29. Thestrip 165 is connected to a tip 169 that projects from the bottom of thetubular body 161. The tip 169 has a length or projects outward from thetubular body a sufficient distance to extend substantially into the slot42 of the yoke 17 in a bone screw assembly mounted to the screwextension assembly 32 that the rod detector passes through. The tip 169thus has a length sufficient so that it will be contacted by a rod 25 asit enters the slot 42 in the yoke.

The flag 154, strip 165 and tip 169 thus form a generally continuousindicator 168 that is pivotably connected to the tubular body 161 at apivot mount 173 as shown in FIG. 30. The pivot mount thus permits theindicator 168 to rock back and forth about the mount 173 from theposition shown in FIG. 29 in which the flag 164 is to the right of thecap 166, and the position shown in FIG. 31 in which the flag 164′ is tothe left of the cap. A slot 167 in the cap 166 accommodates thismovement of the flag. A bias spring 174 bears against the strip 165 topush the strip and flag 164 to the position shown in FIG. 29. Thisposition is the neutral position of the rod detector 160, indicative ofthe absence of a rod within the screw extension assembly 32.

The flag is moved from the neutral position 164 to the positiveindication position 164′ in response to deflection of the tip 169.Movement of the tip to the position 169′ in FIG. 31 is sufficient tocause the flag to shift to the position 164′. The slight movement of thetip is magnified by the pivot mount 174 operating as a fulcrum and thelength of the strip 165 terminating in the flag 164. In one embodimentthe tip 169 is formed as a thin flexible strip of material, such asNitinol, that is capable of bending to the position 169″. The tipdeflects in response to pressure from a connecting rod 25 as shown inFIG. 33. The additional flexibility of the tip 169 allows the tip to belong enough to enter the slot 42 of the yoke 17 and still allow passageof the connecting rod through the screw extension assembly and/or bonescrew yoke.

The rod detector 160 may incorporate elements to enforce properpositioning of the detector relative to the screw extension assembly andto temporarily restrain the detector from removal. Thus, the detectormay include a guide post 170 extending through the slot 162 in thetubular body to pass through a hole 165 a in the strip 165 when thestrip is deflected, as shown in FIG. 31. The guide post 170 carries aspring biased positioning ball 171 that extends outward from the tubularbody 161 opposite the slot 162. This positioning ball 171 is configuredto seat within a positioning groove 176 (see also FIG. 12) defined inthe bore 58 of the inner sleeve 57. This feature provides resistance toremoval of the rod detector 160 from the screw extension assembly. Thebase 166 a of the cap 166 may be configured complementarily to theproximal end 88 of the inner sleeve, such as in a hex configuration.This feature prevents relative rotation between the rod detector and thescrew extension assembly once the detector has been seated within theinner sleeve.

Rod Persuader Assembly

Once a connecting rod 25 is situated at least within the screw extensionassemblies 32 at each instrumented vertebral level, the rod must benestled or seated within the slot 42 of the yoke 17 of each bone screwassembly 15. In the procedures described herein, the rod may be fullyseated in the yoke slot by manipulation of the rod introducer assembly34. This approach is often challenging in part because the rodintroduction site is not readily visible or because there are nosuitable tactile indicators that the rod is properly seated in everybone screw assembly. In order to ensure proper placement of the rod, arod persuader assembly 36 may be mounted on one or more of the screwextension assemblies 32 as illustrated in FIG. 3. Details of the rodpersuader assembly 36 and its operation can be understood from FIGS.34-40.

The persuader assembly includes an outer tube 180 defining a bore 181sized to pass over the outer sleeve 55 of the screw extension assembly32, as shown in FIG. 3. The distal end of the outer tube definesdiametrically opposed scallops 182 that are configured to seat on theouter surface of the connecting rod 25 when the persuader is inoperation. The persuader includes an advancement mechanism 184 driven bya lever 185. The lever 185 may be connected to or integral with acoupling element 186. The coupling element is arranged and configured toengage a persuader coupling member 78 defined on the outer sleeve 55 ofthe screw extension assembly 32. In one embodiment the coupling element186 and coupling member 78 form a rack and pinion arrangement. Thus, asshown in FIG. 35 the coupling element 186 of the rod persuader assembly36 is a pinion gear while the coupling member 78 of the outer sleeve isthe rack. It can thus be appreciated that as the coupling element 186 ispivoted about the pivot hub 188 the pinion gear travels up or down therack, depending upon the direction of rotation. It is noted that thescrew extension assembly includes a coupling member 78 on opposite sidesof the outer sleeve 55. The coupling members are arranged at 90 degreesto the rod slot 59. With this arrangement the scallops 182 of the outertube 180 will contact the connecting rod 25 and the persuader assembly36 may be coupled to the screw extension assembly 32 on either oppositeside.

The advancement mechanism 180 may be provided with a release lever 189that releases a locking mechanism 190 operable to lock the couplingelement 186 and coupling member 78, or rack and pinion, in the positionshown in FIGS. 38-39. In this position, the outer tube 180 has beenadvanced the full length of its travel along the outer sleeve 55 so thatthe scallops 182 contact the connecting rod 25 and force the rod intothe slot 42 of the yoke 17 and/or into the sleeve 18 of the screwassembly 15. As shown in FIG. 39, the release lever 189 is connected toa release element 193 by a linkage 194. A pawl 191 is pivotably mountedto the outer tube 180 to engage the pinion gear or coupling element 186to prevent rotation in one direction while permitting rotation in theopposite direction. The pawl 191 thus prevents rotation of the lever 185upward to the position shown in FIG. 34 but permits rotation downwardfrom the position in FIG. 35 to the position in FIG. 39. The releaseelement 193 includes a prong 195 that is arranged to push or rotate thepawl 191 away from the coupling element 186, thereby allowing theelement (pinion gear) and lever 185 to rotate freely in eitherdirection. Thus, depressing the release lever 189 toward the advancementlever 185 actuates the linkage 194 to push the release element 193toward the pawl 191. The release element 193 may be spring biasedoutward from the hub 188, which in turn biases the locking mechanism 190to the locked position with the pawl 191 in contact with the couplingelement 186.

The rod persuader assembly 36 may include a feature to temporarily holdthe advancement lever 185 in the upward position shown in FIG. 40. Inthis position the rod persuader assembly and especially the scallops 182of the outer tube 180 are offset from the yoke 17 and the rod 25. Thisarrangement may be beneficial in procedures in which the rod persuaderassembly is mounted on a screw extension assembly prior to introductionof a connecting rod. This temporary holding feature may be implementedby a spring-biased ball 196 biased toward a detent 197 in the pivot hub188. The detent 197 is arranged to receive the ball 196 only when thelever 185 is in its upright position. Otherwise the ball simply rolls orslides along the remainder of the pivot hub 188.

Distraction/Compression Instrument

A distraction/compression instrument 200 is illustrated in FIGS. 41-46.The assembly is configured to particularly operate on screw extensionassemblies of bone screws engaged within adjacent vertebrae as shown inFIGS. 47-48. The instrument 200 includes a pair of opposed jaws 202R and202L defining a contractible workspace 203 therebetween. (Thedesignation R and L is arbitrary and merely indicative of likecomponents on opposite sides of a midplane M passing along thelongitudinal axis of the instrument 200 and between the jaws. (Forclarity the R and L designation may not be used when referring to bothjaws 202 together). The jaws 202R, L are generally elongate and parallelto each other and define a plane T extending through the jaws 202R, Land perpendicular to the midplane M, as shown in FIG. 41. The jaws maybe provided with pads 202 a that may be formed of a material adapted tocontact the outer sleeve of a screw extension assembly 32 withoutdamaging the sleeve. The pads 202 a may also be resilient and/orcompressible to modestly embrace the outer sleeves as the jaws 202 aredrawn together.

The jaws are linked to a corresponding pair of handles 204R, L in ascissors-type configuration—i.e., the handle 204R is on the oppositeside of the mid-plane of the apparatus from the corresponding jaw 202R.The handles are pivotably connected at a pivot 205 and with acorresponding linkage arm 206R, L extending beyond the pivot. The twolinkage arms 206 are connected to the corresponding jaws 202 by alinkage mechanism 208 that is configured to allow the jaws 202R, L to bedrawn together with the facing surface of the jaws or the pads 202 aremaining generally parallel to each other and to the mid-plane M of theinstrument. The linkage mechanism 208 includes a cross arm 210R, Lconnecting each linkage arm 206R, L to the corresponding jaw 202R, L, asbest seen in FIG. 42. The cross arms 210 are pivotably connected at apivot 211. The ends of the cross arms 210 are slidably engaged to aguide channel member 213R, L attached to or integral with acorresponding jaw 204R, L. The linkage mechanism 208 is thus configuredto that as the handles 204R, L are squeezed together the cross arms210R, L slide to the end of the guide channel members 213R, L, as shownin FIG. 43. The cross arms 210 also pivot together, thereby drawing thejaws 202 together and closing the workspace to the reduced configuration203′ shown in FIGS. 43, 44.

An adjustable ratchet mechanism 215 is connected between the ends of thehandles 204R, L. The ratchet mechanism is operable to hold the handlesin a plurality of positions ranging from the fully open position shownin FIG. 42 to the fully closed position shown in FIG. 43. A leaf springassembly 116 is disposed between the handles and configured to bias thehandles apart. An adjustable stop 217 may be provided on the ratchetmechanism 215 to adjust the span of the fully open position of thehandles when they are biased outward by the leaf spring assembly 217.Other mechanisms for biasing the handles and/or holding the handles in aparticular position are contemplated.

The compression/distraction instrument 200 includes a fulcrum 218 thatprovides leverage for the compression or distraction of the vertebrae.The fulcrum includes a base 219 that is mounted on a support 220. Oneleg 220R of the support is connected to the jaw 202R while another leg220L is connected to the other jaw 202L. A cross beam 221 is supportedby the two legs 220R, L generally parallel to the plane of movement P ofthe jaws 202, as shown in FIG. 41. The base 219 of the fulcrum 218defines a bore 219 a configured to be slidably mounted on the cross beam221. The beam 221 may be provided with a guide or anti-rotation slot 222that receives one or more pins 223 extending from the fulcrum base 219into the slot. The fulcrum 218 is thus supported in the instrument 200to allow slidable movement of the axis of the fulcrum 218 in a planethat is spaced above and substantially parallel to the plane T. Aplunger or friction pin 226 may be provided in the base 219 that isadapted to frictionally engage or apply pressure to the cross beam 221in a manner sufficient to hold the fulcrum against shifting or wobblingwhile still allowing the fulcrum to slide along the beam. The plunger226 may be adjustable to vary the pressure applied to the cross beam.

The cross beam 221 is affixed or attached to one of the legs, leg 220Lfor instance. The other leg, leg 220R in this example, includes a collar224 defining an opening 225 to slidably receive the cross beam 221.Thus, as the jaws 202 move together the collar 224 and more specificallythe leg 220R slides along the cross beam, as seen by comparing FIGS. 41and 43.

As shown in FIGS. 41-45, the fulcrum 218 is in the form of a generallyelongate cylindrical rod having an effective width W₁. The width of thefulcrum impacts the manner in which the vertebrae aredistracted/compressed. Thus, in one aspect, the instrument 200 may beprovided with additional fulcrums having different configurations andwidths. For instance, the fulcrum 227 shown in FIG. 46 a is alsocylindrical but has an effective width W₂ that is less than the width W₁of the fulcrum 218. Alternatively the fulcrum can be generallyrectangular with rounded sides, like the fulcrum 228 and 229 in FIGS. 46b and 46 c, respectively. The two fulcrums may have differing widths W₃and W₄ that may also differ from the widths W₁ and W₂. In each of theillustrated embodiments the fulcrums present a rounded surface tocontact the screw extension assemblies. The rounded surface facilitatespivoting of the extension assemblies about the fulcrum as describedbelow.

As its name suggests, the compression/distraction instrument 200 may beused to selectively compress or distract adjacent vertebrae that areinstrumented with the bone screw assemblies 15 and connecting member/rod25. Whether the instrument is used to compress or distract depends uponthe orientation of the fulcrum, such as fulcrum 228, relative to thejaws 202. Thus, as shown in FIG. 47, the instrument 200 is arranged forcompression with the fulcrum 228 above the jaws 202 or, in other words,with the jaws 202 disposed between the fulcrum and the connecting rod25. In this orientation, when the handles are manually squeezed togetherthe jaws 202 pivot toward each other in the direction of the arrows P.Since the jaws bear against the screw extension assemblies 32 below thefulcrum 228 the extension assemblies pivot about the fulcrum toward eachother in the direction of the arrows C. This movement draws the screwassemblies 15 together along the connecting member 25, therebycompressing the adjacent vertebrae to which the screw assemblies areengaged.

When distraction is desired the instrument 200 is inverted—i.e., turnedover—so that the fulcrum 228 is between the jaws 202 and the screwassemblies 15, as shown in FIG. 48. In this orientation when the jawsare moved toward each other in the direction P the extension assemblies32 pivot about the fulcrum so that the distal or lower portion of theassemblies flare outward in the direction of the arrows D. This movementof the extension assemblies slides the screw assemblies 15 along the rod25, thereby distracting the adjacent vertebrae.

It can be appreciated that the amount of distraction or compression islimited by the angle through which the screw extension assemblies maypivot before contacting each other. For instance, in the compressionmode of FIG. 47, the screw extension assemblies 32 will contact eachother around the middle of the connecting rod 25. In the distractionmode of FIG. 48, the proximal ends of the assemblies, such as theproximal ends 55 a of the outer sleeves of the assemblies, will contacteach other when the extension assemblies have pivoted far enough outwardin the direction D. The amount of angular movement of the screwextension assemblies that occurs before this contact is affected by thewidth of the fulcrum. Increasing the width of the fulcrum increases theamount of angular pivoting, and conversely decreasing the fulcrum widthdecreases the range of extension assembly pivoting.

In addition, the location of the fulcrum along the length of theextension assemblies 32 will also affect the maximum available pivotangle. In the compression mode of FIG. 47, the closer the fulcrum ismoved to the screw assemblies 15 or to the surgical incision S thegreater the angular range of motion. Conversely, in the distraction modeof FIG. 48, the angular range of motion increases as the fulcrum ismoved farther from the screw assemblies or incision.

As shown in FIG. 45 the fulcrum, such as fulcrum 218, may be slidablyoffset from the mid-plane M of the instrument 200. Upon actuation of theinstrument the fulcrum will slide along the cross beam 220 as thefulcrum successively contacts the extension assemblies. The sliding ofthe fulcrum 218 and the use of fulcrums having different widths allowthe surgeon more flexibility in handling different sized and spacedvertebrae in patients.

Percutaneous Surgical Procedures

The instruments disclosed herein may be used to percutaneously introducepedicle screws and a connecting member for multiple level fixation ofthe spine. The instruments may be used in several different approachesas described with reference to FIGS. 49-57. In each approach the pedicleof the patient is accessed according to known techniques. Guide wiresmay be used to locate the pedicle of each vertebra to be instrument andto facilitate the subsequent introduction of tools, instruments andimplants. Once the guide wires are properly positioned a series ofseparate incisions I are created to provide a pathway to each pedicle.Thus, in one approach a series of tissue dilators and/or tissueretractors may be introduced over each guide wire to create the pathwayto the each pedicle. A final dilator or tissue retractor may remain inposition to create the working channel for introduction of the pediclescrew assembly 15 into the corresponding pedicle. The size or diameterof the working channel may be larger if the bone screw assembly is to beintroduced with a rod persuader assembly mounted to a screw extensionassembly.

Once the working channel pathway has been created the pedicle isprepared in a known manner for introduction of a bone screw. Thus, thepedicle may be cannulated by a safety awl and then tapped to a suitabledepth. A bone screw assembly 15 is engaged to a screw extension assembly32 as described above and as shown in FIGS. 10-11. With the extensionassembly in its locked configuration (see FIG. 16) the yoke 17 of thebone screw assembly 15 is tightly held by the screw extension assembly32. In some procedures it may be desirable to also mount a rod persuaderassembly 36 onto the screw extension assembly 32 as shown in FIGS. 3 and36. In this instance, the persuader assembly may be locked onto theextension assembly with the advancement mechanism 184 and lever 185 inthe position shown in FIG. 37. At this point the instruments are not ina position to receive a connecting rod so there is no need to retractthe persuader assembly distal end from the vicinity of the yoke.

Once the screw extension assembly (and alternatively the rod persuaderassembly) is engaged to the bone screw assembly 15 the screw driverassembly 100 may be advanced through the bore 58 of the inner sleeve 57of the extension assembly 32, as depicted in FIG. 19. The shaft 102 maybe advanced entirely through the extension assembly and into the screwassembly until the engagement end 103 is seated within the toolengagement recess 22 of the bone screw. The entire assembly, bone screwfirst, is then advanced along the previously placed guide wire until thebone screw 16 of the screw assembly arrives at the tapped opening in thepedicle. The screw driver assembly 100 may then be used to drive thebone screw into the pedicle until seated. The screw extension assembly32 may be held by the surgeon while the pedicle screw assembly 15 isdriven by the screw driver assembly 100 into the pedicle. When the lowersurface of the head 16 a of the bone screw seats in the pedicle, thelower portion of the yoke is spaced above the surface of the vertebraallowing unhindered articulation of the yoke and pedicle screw extensionassembly 32 which is tightly affixed to the yoke. The depth andpositioning of the bone can be verified in a known manner.

Once the pedicle screw position has been verified the screw driverassembly and guide wire may be removed. In a procedure that does notutilize an initially placed rod persuader assembly, the bone screwassemblies 15 and screw extension assemblies 32 will appear as shown inFIG. 49 with each assembly extending through its own incision I. In aprocedure in which a persuader is initially placed on at least one screwextension assembly 32 the surgical site will appear as in FIG. 52. Thescrew extension assemblies 32 may be used to gage the size of theconnecting member or rod 25 required to span the instrumented vertebrae.Thus, a known caliper instrument (not shown) may be seated on theoutermost extension assemblies to indicate the desired rod length. Theproper length rod is selected and contoured as desired. As previouslydiscussed, the pre-bent rod 25 disclosed herein may be used in oneorientation for correcting or creating lordosis and in the oppositeorientation for correcting or creating kyphosis. More complex bends maybe introduced into the connecting rod using a suitable rod bender.

The selected rod is grasped by the rod introducer assembly 34 as shownin FIG. 25 and as described above. As to the proper positioning of thecurvature of the rod, for a lordotic curve the rod should curve towardthe handle 110 of the introducer assembly, as shown in FIG. 49. For akyphotic curve the rod should curve in the opposite direction away fromthe handle. Once the rod orientation has been verified the secondlocking mechanism 150 of the introducer assembly 34 may be engaged tohold the rod in its “neutral” position, as depicted in FIG. 22. In theneutral position the rod is retained by the introducer assembly but maypivot about the engagement posts 120, as described above in relation toFIGS. 22 and 26. The desired angle of the rod 25 relative to the outersleeve 114 of the rod introducer assembly 34 may be set and the rodlocked by engaging the first lock 140 upon fully depressing the lever113 (see FIG. 21). In many procedures the rod is initially situated at a45 degree angle to the outer sleeve 114, as shown in FIG. 49. If it isfound during the procedure that a different rod angle is needed, thepush button 145 may be depressed to release the first lock 140 andplacing the introducer assembly in the neutral position to permitadjustment of the rod angle. Once the new rod angle has been set thelever 113 may be depressed to engage the first lock and tightly grip therod again.

With the screw assemblies threaded into the pedicles with the screwextensions attached the connecting rod can then be introduced. Incertain procedures one or more rod detectors 160 may be placed withinone or more screw extension assemblies 32, as illustrated in FIG. 33. Inone procedure the rod is introduced through the incision at an extremecephalad or caudal one of the screw extension assemblies, as depicted inFIG. 49. In this depiction the rod 25 is oriented at a 45 degree angleto the outer sleeve 114 of the rod introducer 34. The introducer ismanipulated so that the leading end 27 of the rod 25 passes through theslot 59 of the extension assembly and subsequently or simultaneouslythrough incision I. The slot 59 may act as a guide to slide the distalend 27 of the rod downward through the incision. If necessary the angleof the rod may be adjusted as described above to facilitate entry of therod through the incision.

Once below the fascia S the rod can be advanced subcutaneously beneaththe fascia toward the other screw assemblies. The sides of the rod slots59 and 67 in the outer and inner sleeves, respectively, of the extremescrew extension assembly can further act as a guide to keep the rod 25aligned with the rod slots in the other extension assemblies. As the rodenters the rod slots of the successive extension assemblies theindicator flag 164 (FIG. 33) of the associated rod detector will shiftpositions to indicate that the rod is within the respective slot. Whenthe rod is fully positioned within each of the screw extensionassemblies 32 the outer sleeve 114 of the rod introducer 34 may abut theouter sleeve 55 of the extreme extension assembly, as shown in FIG. 50.The rod detectors may then be removed.

At this point it is desirable that the rod be oriented at a 90 degreeangle to the outer sleeve 114 of the introducer, as illustrated in FIG.51. Adjustment of the rod angle can be accomplished by depressing thepushbutton 145 to release the second lock and allow the rod to bepivoted relative to the outer sleeve 114. It can be pointed out that dueto the construction of the rod introducer the surgeon will receive atactile indication produced by the rod introducer when the rod hasdislodged from the current angular position and re-seated in the newposition. In accordance with this particular procedure the rodintroducer 34 is the only tool required to seat the rod within the yokesin anticipation of locking the screw assembly with a set screw or aclamping mechanism in accordance with the design of the screw assembly.Consequently, once the rod is fully seated the rod introducer 34 may bedisconnected from the rod 25 by depressing the pushbutton 145 to releasethe second lock and then depressing the release buttons 152 to releasethe first lock. The actuation lever 113 may then be pivoted outward fromthe handle 110, as shown in FIG. 51 to spread the flexible legs of theouter sleeve and release the legs from the engagement end 28 of the rod(see FIG. 20). The rod persuader is then withdrawn through the incision.

In an alternative procedure, one or more rod persuaders 36 may beselectively used to seat the rod 25 within the bone screw assemblies 15,as shown in FIGS. 52-54. The rod persuader 36 may be introduced throughthe incision I with the screw extension assembly 32, as described above,or at the discretion of the surgeon after the screw assemblies andextension assemblies have been engaged to the vertebrae. One or more roddetectors may be positioned as described above. Prior to introducing therod 25 the rod persuader(s) must be in the retracted position shown inFIG. 52 to avoid interfering with the rod as it enters the rod slots inthe screw extension assemblies. Thus, the advancement lever 185 of theassembly 36 is in its upward position. The spring biased ball and detentstructure discussed above (FIG. 40) will hold the lever and thus theouter tube 180 in the retracted position.

As shown in FIG. 52 the rod 25 is introduced through the incision I androd slot 59 at the extreme cephalad or caudal screw assembly 15 andextension assembly 32. Once the rod has been fully advanced through eachof the extension assemblies (with the outer sleeve 114 of the introducer34 abutting the outer sleeve 55) the rod introducer assembly 34 may bemoved to the neutral position and the rod persuader assembly 36 can beactuated. The lever 185 is pivoted downward, which drives the outersleeve 180 downward so that the rod scallops 182 seat on the rod 25, asillustrated in FIG. 53. The advancement lever 185 is pivoted to itslowermost position to drive the outer sleeve 180 fully downward, asshown in FIG. 54. In this position the outer sleeve has pushed the rod25 to fully seat within the yokes of the screw assemblies. The rodintroducer 34 may remain engaged to the rod 25 during this process. Oncethe rod is fully seated the introducer 34 may be disengaged from the rodas explained above.

In the procedures just described the rod is introduced exteriorly of theextension assemblies through an incision common with an outermost screwand extension assembly. In an alternative procedure the rod isintroduced into the surgical site through a separate incision I_(rod),as illustrated in FIGS. 55-56. This separate incision I_(rod) may beoriented at a 45 degree trajectory with respect to the extreme cephalador caudal screw extension assembly through which the rod is firstintroduced. The rod 25 is preferably at the 45 degree orientationrelative to the rod introducer 34 as shown in FIG. 55. As shown in FIG.55 the outer sleeve 114 will pass through the separate incision I_(rod)to guide the rod subcutaneously through each successive extensionassembly. With this procedure it may be desirable to position a roddetector within each screw extension assembly to provide a visualindication when the rod enters each assembly.

As shown in FIGS. 55-56 each extension assembly may be provided with arod persuader assembly 36. Prior to introducing the rod 25 theadvancement levers 185 of all the rod persuader assemblies are in theirfully retracted positions. Once the rod is in place the levers arepivoted downward to the respective outer sleeves 180 downward to seatthe rod in the corresponding yoke. It can be appreciated that theadvancement levers may be pivoted simultaneously or sequentially orpartially rotated in steps, all with the goal of smoothly seating therod within each screw assembly 15.

Another procedure approach is shown in FIG. 57. In this approach acommon incision I_(C) is formed between the separate incisions throughwhich the screw and extension assemblies have been advanced. The rod 25may be introduced through the rod slots 59 of each extension assembly 32above the fascia S under direct vision, so that rod detectors are notrequired. In this approach the rod persuader(s) are not mounted on thescrew extension assemblies until after the rod has been properlypositioned within the assemblies.

Once the rod has been positioned above the fascia the rod introducerassembly 34 can be manipulated to push the rod through the incisionI_(C) to the position shown in FIG. 57. If necessary the rod anglerelative to the outer sleeve 114 may be adjusted, as described above.The rod may be fully seated within the screw assemblies 15 with orwithout the rod persuader assemblies.

In each approach, once the rod has been fully seated within the screwassemblies the set screw or locking element may be advanced through eachscrew extension assembly to engage the respective bone screw assembly.In some instances the screw assemblies are finally tightened onto therod. In other instances compression or distraction may be necessary. Inthese instances the bone screw assemblies may be provisionally tightenedin a manner that permits one or more of the screw assemblies to slidealong the connecting rod. The compression/distraction device 200 may beused as described above to perform the necessary adjustments to thescrew assemblies, after which the assemblies may be finally tightened.After the rod and screw fixation construct is complete the screwextension assemblies can be removed and the incisions closed.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe invention are desired to be protected.

1. A bone screw extension assembly for use in placing a connecting rodpercutaneously into tissue of a patient, comprising: a multi-axial bonescrew including a shaft having a threaded screw portion and a yokearticulatingly attached to said shaft, said yoke having an exteriorsurface and a slot therethrough for receiving a connecting rod, saidslot defined by a pair of upstanding arms having opposed interiorsurfaces, said yoke including an extension coupling surface; a screwextension releasably coupled to yoke of said bone screw, said screwextension including an elongate hollow outer sleeve having a distal endand a proximal end and an elongate hollow inner sleeve disposed withinsaid outer sleeve and having a distal end and a proximal end, said outersleeve and said inner sleeve being movable rotationally relative to eachother; and a screw engaging member projecting radially from the distalend of one of said outer sleeve and said inner sleeve and configured toengage said extension coupling surface upon relative rotation of saidinner sleeve and said outer sleeve; and a securement member includingcooperative surfaces defined adjacent the distal end of said innersleeve and said outer sleeve operable upon relative rotation of saidinner sleeve and said outer sleeve to radially secure said inner sleeveand said outer sleeve together and to maintain the engagement of saidscrew engaging member and said bone screw.
 2. The bone screw extensionassembly of claim 1, wherein said screw engaging member is disposed onsaid inner sleeve and projects radially outwardly therefrom.
 3. The bonescrew extension assembly of claim 2, wherein said screw engaging memberincludes a flange having a thickness.
 4. The bone screw extensionassembly of claim 3, wherein the thickness of said flange increasesradially outwardly.
 5. The bone screw extension assembly of claim 2,wherein: the cooperative surface of said securement member on said innersleeve includes a flange extending radially outwardly from said innersleeve; and the cooperative surface of said securement member on saidouter sleeve is defined by a groove extending into an inner surface ofsaid outer sleeve.
 6. The bone screw extension assembly of claim 5,wherein: said flange includes a portion projecting toward the distal endof said inner sleeve; and said groove opens upwardly toward the proximalend of said outer sleeve and is configured to receive said distallyprojecting portion of said flange.
 7. The bone screw extension assemblyof claim 3, wherein the extension coupling surface of said yoke isdisposed on at least one of said interior surfaces of said upstandingarms.
 8. The bone screw extension assembly of claim 7, wherein saidextension coupling surface of said yoke is defined by an undercutextending radially into said at least one interior surface of saidupstanding arms, said undercut being configured to receive the flange ofthe screw engaging member.
 9. The bone screw extension assembly of claim8, wherein said undercut extends into the interior surfaces of bothupstanding arms.
 10. The bone screw extension assembly of claim 9,wherein: said outer sleeve includes a perimetric sidewall defining abore through which said inner sleeve extends; and a pair of opposingslots is formed through said sidewall, each opening at the distal end ofsaid outer sleeve and extending for a length proximally, said slots ofsaid outer sleeve being aligned with the slot of said yoke.
 11. The bonescrew extension assembly of claim 10, wherein: said inner sleeveincludes a perimetric sidewall defining a bore; and a pair of opposingslots is formed through said sidewall, each opening at the distal end ofsaid inner sleeve and extending for a length proximally.
 12. The bonescrew extension assembly of claim 11, wherein a pair of opposing saidflanges is disposed between the pair of opposing slots of said innersleeve.
 13. The bone screw extension assembly of claim 12, wherein saidscrew extension is couplable to said yoke in an unlocked first positionin which said slots of said outer sleeve are aligned with said slot ofsaid yoke and said slots of inner sleeve are not aligned with andthereby close the slot of said yoke.
 14. The bone screw extensionassembly of claim 13, wherein said screw extension is couplable to saidyoke in a locked second position upon the rotation of said inner sleeverelative to said outer sleeve, in which said screw engaging flanges arereceived in the undercuts in the interior surfaces of said upstandingarms and the slots of said inner sleeve are aligned with the slots ofsaid outer sleeve and the slot of said yoke in a manner to receive aconnection rod.
 15. The bone screw extension assembly of claim 1,further comprising a lock to selectively prevent relative rotationbetween said outer sleeve and said inner sleeve.
 16. The bone screwextension assembly of claim 15, wherein said lock includes adisplaceable retention ring supported between said outer sleeve and saidinner sleeve and operable in a first position to allow free rotationalrelative movement between said outer sleeve and said inner sleeve and ina second position to prevent such movement.
 17. The bone screw extensionassembly of claim 16, wherein said retention ring is spring biased to anormally locked position.
 18. An anchor extension for releasablycoupling to a spinal bone anchor; comprising: an elongate hollow outersleeve having a distal end and a proximal end; an elongate hollow innersleeve disposed within said outer sleeve and having a distal end and aproximal end, said outer sleeve and said inner sleeve being movablerotationally relative to each other; an anchor engaging memberprojecting radially from the distal end of one of said outer sleeve andsaid inner sleeve for engaging the bone anchor upon relative rotation ofsaid inner sleeve and said outer sleeve; and a securement memberincluding cooperative surfaces defined adjacent the distal end of saidinner sleeve and said outer sleeve operable upon relative rotation ofsaid inner sleeve and said outer sleeve to radially secure said innersleeve and said outer sleeve together and to maintain the engagement ofsaid screw engaging member and said bone screw.
 19. The anchor extensionassembly of claim 18, wherein said anchor engaging member is disposed onsaid inner sleeve and projects radially outwardly therefrom.
 20. Theanchor extension assembly of claim 18, wherein said screw engagingmember includes a flange having a thickness.
 21. The anchor extensionassembly of claim 19 wherein the thickness of said flange increasesradially outwardly.
 22. The anchor extension assembly of claim 19,wherein: the cooperative surface of said securement member on said innersleeve includes a flange extending radially outwardly from said innersleeve; and the cooperative surface of said securement member on saidouter sleeve is defined by a groove extending into an inner surface ofsaid outer sleeve.
 23. The anchor extension assembly of claim 22,wherein: said securement member flange includes a portion projectingtoward the distal end of said inner sleeve; and said securement membergroove opens upwardly toward the proximal end of said outer sleeve andis configured to receive said distally projecting portion of saidsecurement member flange.
 24. In combination for use during spinalsurgery, a multi-axial bone screw including a shaft having a threadedscrew portion, a screw head and a yoke articulatingly attached to saidscrew head, said yoke having an exterior surface and a slot therethroughfor receiving a connecting rod, said slot defined by a pair ofupstanding arms having opposed interior surfaces, said yoke having anextension coupling surface thereon; an elongate hollow screw extensionhaving a distal end and a proximal end, the extension including a screwengaging member at the distal end thereof in engagement with saidextension coupling surface on said yoke, said extension having a pair ofopposing extension slots opening at the distal end of said extensionaligned with said yoke slot and extending for an extent toward theproximal end of said extension, said extension including on an outersurface thereof a persuader coupling member; and a rod persuaderincluding an elongate hollow sleeve extending over the outer surface ofsaid screw extension and releasably couplable to said persuader couplingmember, said persuader sleeve having a distal end defining a rodengaging surface disposed in a first position to allow a rod to beintroduced through the opposing slots in said screw extension andoperably movable in cooperation with said persuader coupling member to asecond position wherein said rod engaging surface extends over at leasta portion of said extension slots to urge said rod into the slot of saidyoke.
 25. The combination of claim 24, wherein said rod persuaderincludes a coupling element configured to releasably engage saidpersuader coupling member.
 26. The combination of claim 25, wherein saidcoupling element and said persuader coupling member form a rack andpinion arrangement.
 27. The combination of claim 26, wherein saidcoupling element is defined by a pinion gear and said coupling memberdefines a rack.
 28. The combination of claim 27, wherein said rodpersuader further includes an advancement mechanism.
 29. The combinationof claim 28, wherein said advancement mechanism includes a leverconnected to said coupling element.
 30. The combination of claim 24,wherein said persuader coupling member is disposed on said extension atan orientation approximately ninety degrees with respect to saidextension slots.
 31. The combination of claim 30, wherein said extensionincludes a pair of persuader coupling members oppositely arrangedthereon and oriented approximately ninety degrees with respect to saidextension slots for selective coupling of said persuader on oppositesides of said extension.
 32. In combination for use during spinalsurgery, a multi-axial bone screw including a shaft having a threadedscrew portion, a screw head and a yoke articulatingly attached to saidscrew head, said yoke having an exterior surface and a slot therethroughfor receiving a connecting rod, said slot defined by a pair ofupstanding arms having opposed interior surfaces, said yoke having anopening communicating with said screw head and including an extensioncoupling surface on the interior surfaces of said upstanding arms; anelongate hollow screw extension having a distal end and a proximal end,the extension having a rotationally movable screw engaging memberprojecting radially outwardly from the distal end thereof in engagementwith said extension coupling surface of said yoke arms; and a screwdriver including an elongate inner shaft having a distal end and aproximal end and an outer retention sleeve disposed about said innershaft and having a distal end and a proximal end, the inner shaft beingfreely rotatable within said outer retention sleeve and having at itsdistal end a tip extending through said yoke opening in releasableengagement with the head of said bone screw for joint rotationalmovement therewith, the distal end of said outer retention sleeve beingreleasably joined to said yoke for joint movement therewith.
 33. Thecombination of claim 32, wherein the interior surfaces of saidupstanding arms are threaded.
 34. The combination of claim 33, whereinthe distal end of said outer retention sleeve is threaded exteriorly tothreadedly engage the interior threads on said upstanding arms forreleasably holding said outer retention sleeve to said yoke.
 35. Thecombination of claim 32, wherein: said extension includes a pair ofopposed extension slots opening at the distal end thereof and alignedwith said yoke slot, said extension slots extending for an extent towardthe proximal end of said extension, said extension including on an outersurface thereof a persuader coupling member, and said combinationfurther includes a rod persuader including an elongate hollow sleeveextending over the outer surface of said screw extension and releasablycoupled to said persuader coupling member, said persuader sleeve havinga distal end defining a rod engaging surface disposed in a firstposition to allow a rod to be introduced through the opposed slots insaid screw extension and operably movable by a mechanism on saidpersuader sleeve cooperating with said persuader coupling member to asecond position wherein said rod engaging surface extends over at leasta portion of said extension slots to urge said rod into the slot of saidyoke.